<body>
    <style>
        /* html字号默认 16px */
        .box {
            /* 16 * 5 = 80px */
            width: 5rem;
            /* 16 * 2 = 32px */
            height: 2rem;
            background-color: skyblue;
        }
    </style>

    <div class="box"></div>
</body>